Injection moulding may be used to make a wide variety of articles including articles having relatively complex shapes and a range of sizes. Injection moulding is, for instance, suited to the manufacture of articles used as caps and closures for food and drink applications, such as for bottles containing carbonated or non-carbonated drinks, or for non-food applications like containers for cosmetics and pharmaceuticals.
Injection moulding is a moulding process in which a polymer is melted and then filled into a mould by injection. During initial injection, high pressure is used and the polymer melt is compressed. Thus, upon injection into the mould the polymer melt initially expands or “relaxes” to fill the mould. The mould, however, is kept at a lower temperature than the polymer melt. As the polymer melt cools, shrinkage tends to occur. To compensate for this effect, back pressure is applied. Thereafter the polymer melt is cooled further to enable the moulded article to be removed from the mould without causing deformation.
An important property of an injection moulded article is its stress crack resistance. It will be appreciated that the injection moulded articles of the invention should not exhibit brittle failure and should therefore possess a high stress crack resistance. The present inventors therefore sought new HDPEs, developed in particular for the cap and closure market, which possess improved stress cracking resistance.
The HDPE should also offer injection moulded articles with high impact strength so the articles withstand being dropped and withstand transportation.
It is also important that produced articles (e.g. caps or closures) having sufficient mechanical strength (rigidity) implying that the HDPE resin should have sufficient stiffness.
Improvements in these mechanical properties must not be at the expense of processability of the polymer. Processability must be maintained or even improved to meet customer needs. Injection moulded articles are produced rapidly and any reduction in processability can increase cycle times and hence reduce process efficiency. To achieve high production speed and hence a process with improved economics, it is important for the resin to have very good flow, in particular flow under pressure (i.e. good rheology).
It is therefore required that the HDPE resin which is used to produce beverage cap or closure has a balance of mechanical and rheological properties. The improvement of one of the properties above however, leads to a reduction in another equally valuable property. For example, stiffness can be increased by increasing polymer density. Unfortunately this will reduce the resistance to crack propagation. Likewise, resin flow can be improved by increasing its melt flow rate but only at the expense of impact resistance. Therefore, it is not easy to achieve the desired balance required properties.
The invention also relates to the preparation of compression moulded articles. Compression moulding is a method of moulding in which the molding material, generally preheated, is first placed in an open, heated mould cavity. The mould is closed, pressure is applied to force the material into contact with all mould areas, while heat and pressure are maintained until the moulding material has set. In injection molding processes, the polymer melt flows under high shear rates and flow ability measured by spiral flow provides a good indication on how the polymer will flow in the mold. In compression molding processes the situation is different. As the polymer melt is not subjected to high shear rates, polymer flow is proportional to MFR of the resin. It is possible that a low MFR HDPE has high spiral flow (due to high shear thinning) but this will not necessarily translate to good flow in a compression molding process. In this respect, the high melt flow of the inventive examples is beneficial to achieve good flow in a compression molding process while high spiral flow gives good flow ability in an injection molding process. Many prior art examples fail to achieve such combination.
The present inventors have therefore devised a narrowly defined multimodal HDPE copolymer that possesses excellent mechanical properties, good flow for both compression and injection moulding and high processability.
Some bimodal HDPE compositions are known. In EP-A-1,565,525 the inventors describe a bimodal HDPE, preferably for blow moulding applications. It has a high molecular weight component which possesses high short chain branching. That is achieved however using a single site catalyst and results in a narrow Mw/Mn. The narrow Mw/Mn makes these polymers less processable and they have poor flow.
WO2010/022941 describes an HDPE for injection moulding possessing good ESCR and flow. The polymer is however, made using an obscure dualsite catalyst and relies on a catalyst comprising a single site catalyst and an iron based catalyst. Such a catalyst is not favoured industrially, as a process involving a dualsite catalyst is necessarily limiting as the conditions cannot be varied between polymerisation stages. The formed polymer in '941 is one based on two copolymer fractions and it is impossible to prepare a homopolymer/copolymer using a dualsite catalyst as described in '941.
WO2013/045663 describes bimodal HDPE's with low melt index with good rigidity and ESCR. These can be used for caps and closures. Unusually, the polymer is made with the lower density component in the first stage of the process, that component having low melt index. In the examples, no experimental conditions or comonomers are mentioned, although the final blend has a low MFR2.
WO2014/180989 describes multimodal polymers for cap and closure manufacture with excellent stress crack and tensile properties that lead to a reduction of angel hair and high tips on forming caps. These polymers have low MFR.
JP4942525 describes a polyethylene resin composition for a bottle cap having an excellent rigidity and excellent high speed molding property without reducing environmental stress cracking resistance. The polymer is a multimodal HDPE having homopolymer and copolymer components and an MFR of 5.0 to 10.0 g/10 min, a density is 0.960 to 0.967 g/cm3, and an Mw/Mn of 8.0 to 12.0.
WO03/039984 describes screw caps made from a bimodal polyethylene in which an ethylene homopolymer is combined with an ethylene copolymer fraction. The caps possess good ESCR, injectability and impact resistance. The polyethylenes therein are generally of lower MFR2 than we require and are primarily targeted at caps for carbonated beverages. Such caps require very high ESCR which is achieved by keeping melt flow and densities at lower level.
The present inventors have found that a particular combination of properties leads to an ideal balance of mechanical properties, rheological properties and processability. By manufacturing a multimodal HDPE with high melt index having an ethylene homopolymer lower molecular weight fraction in combination with a higher molecular weight copolymer fraction gives excellent properties. The invention has been compared to a broad range of commercial moulding HDPE grades of comparable densities to show that the relationships in claim 1 are not ones which can be found in commercial polymers and which yield the advantageous properties highlighted above.
The combination of tailored flow, good stiffness, good rheology and good stress crack is achieved using a blend in which the LMW ethylene homopolymer has very high melt index (and hence low Mw) and high density combined with a HMW component that results in an overall density and MFR2 which is higher than some conventional solutions. Surprising, despite the higher density we maintain high ESCR which might be expected to fall. Our flow is also in the ideal range for injection moulding. The invention is ideally suited to the manufacture of caps for non-carbonated beverage containers as the inventive compositions provide very good balance between flow ability, stiffness and stress cracking resistance.